Free-carrier effects and optical phonons in GaNAs/GaAs superlattice heterostructures measured by infrared spectroscopic
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Internet Journal Nitride Semiconductor Research
Free-carrier effects and optical phonons in GaNAs/GaAs superlattice heterostructures measured by infrared spectroscopic ellipsometry J. ·ik14, M. Schubert12, T. Hofmann2 and V. Gottschalch3 1Center for Microelectronic and Optical Materials Research and Department of Electrical Engineering, University of Nebraska-Lincoln, 2Fakultaet fuer Physik und Geowissenschaften, Arbeitsgruppe Halbleiterphysik,University Leipzig, 3Fakultaet fuer Chemie und Mineralogie, University Leipzig, 4Department of Solid State Physics and Laboratory of Thin FIlms and Nanostructures, Faculty of Science, Masaryk University Brno,
(Received Thursday, March 2, 2000; accepted Friday, April 21, 2000)
The infrared-optical properties of GaAs/GaNxAs1-x superlattice (SL) heterostructures (0 < x < 3.3%) are studied by variable angle-of-incidence infrared spectroscopic ellipsometry (IRSE) for wavenumbers from 250 cm-1 to 700 cm-1. The undoped SL structures where grown on top of a 300 nm thick undoped GaAs buffer layer on Te-doped (001) GaAs substrates by metal-organic vapor phase epitaxy (MOVPE). We observe the well-known Berreman-polariton effect within the GaAs LO-phonon region. We further observe a strong polariton-like resonance near the coupled longitudinal-optical plasmon-phonon frequency of the Te-doped substrate at 306 cm-1. For analysis of the IRSE data we employ the harmonic oscillator dielectric function model and the Drude model for free-carrier response. The additional resonance feature is explained by pseudo surface polariton (PSP) interface modes between the Te-doped GaAs and the undoped GaAs buffer layer / SL film. We find that the PSP modes are extremely sensitive to free-carrier properties within the SL structures, and we obtain a strong increase in free-carrier concentration within the GaNAs SL sublayers with increasing x from analysis of the IRSE data. We further observe the localized vibrational modes of nitrogen at 470 cm-1 in the GaNxAs1-x SL sublayers with a polar strength that increases linearly with x, and which can be used to monitor the nitrogen concentration in GaNxAs1-x.
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Introduction
The nitrogen-doped low band-gap A(III)-B(V) alloys of GaNxAs1-x have recently attracted much attention due to wide-range band-gap properties. The large field of possible optoelectronic applications includes optical interconnections, fast switching systems, and low-band-gap detectors [1]. The large chemical and size differences between N and As causes strongly nonlinear dependence of the band-gap on the composition in this alloy system, and anomalously large optical bowing coefficients have been predicted [2], [3]. Due to the strong lattice mismatch between GaAs and GaNxAs1-x for increasing nitrogen incorporation the critical thickness for pseudomorphic GaNxAs1-x growth decreases rapidly.
SL structures to compensate or stabilize mismatchinduced strain in GaNxAs1-x epilayers may present a way out of the critical-thickness limitation for single epilayers. Studies of the optical propertie
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